1. Field of the Invention
The present invention relates to an image pickup device, and more particularly to focusing based on frequency analysis of image information.
2. Related Background Art
Image pickup device, employing an image sensor and adapted for use in a video camera or the like, is usually equipped with an automatic focusing device for achieving focusing of an object. Among known auto focusing device is one utilizing the signal from the image sensor to effect control by a circuit separate from the signal processing circuit for forming a video signal or the like, and for analyzing the frequency of the image signal and focusing the lens so as to maximize the high frequency components.
On the other hand, digital processing of the image (video) signal has become popular in recent years.
The digitized video signal has an advantage that the image quality is not deteriorated even after repeated electromagnetic conversion (recording and reproduction), but mere digitization of the video signal results in an enormous amount of information. In the conventional image pickup device, therefore, signal compression is conducted, utilizing the redundancy of the video signal, in order to reduce the required memory capacity or the signal transfer time.
There are already known various methods for such signal compression, but DCT (discrete cosine transformation, utilizing the real part of high-speed Fourier conversion) is increasingly utilized, also in consideration of ease of hardware preparation.
The DCT involves spatial frequency analysis of the image, namely evaluation of the image sharpness.
Because of the situations explained above, the conventional image pickup device with automatic focusing function and digital signal processing function has to be provided with an automatic focusing circuit and a circuit for digital signal processing such as A/D converter, compression circuit etc.
However, if digital processing of image signal is intended in such conventional image pickup device, there are required, separately, a circuit for automatic focusing and a circuit for such digital signal processing, so that the magnitude of circuitry becomes inevitably large, leading to a larger dimension or a higher cost of the video camera etc. Also an increased magnitude of circuitry increases the power consumption, thus leading to a shorter time of operation in an equipment such as video camera, which is frequently used outdoors with a battery.